1. Technical Field
This invention relates to a wide scan quasioptical frequency diplexer, and more particularly, to using an array of waveguide sections as a diplexer where the input and output ports of the array are tilted at oblique angles with respect to its longitudinal axis, the tilted ports allowing the diplexer to operate over a wide angle of scan.
2. Description of the Prior Art
In order to achieve greater utilization of microwave antenna systems, frequency diplexing is needed to allow simultaneous transmission and reception of microwave signals. One method of frequency diplexing is to incorporate a waveguide diplexer with the antenna feed. Alternatively, the incoming beam may be intercepted by a frequency sensitive device before it enters the feed, this method being referred to as quasi-optical diplexing.
A number of designs have been suggested in the past for quasi-optical diplexing at microwave frequencies. One such design technique is discussed in the article "A Quasi-Optical Polarization-Independent Diplexer for Use in the Beam Feed System of Millimeter-Wave Antennas" by A. A. M. Saleh et al in IEEE Transactions on Antennas and Propagation, Vol. AP-24, No. 6, November 1976 at pp. 780-785. This paper presents a diplexer consisting of a parallel-plane Fabry-Perot resonator having two metallic meshes with rectangular cells. The ratio between the width and length of the rectangles is chosen to yield polarization-independent operation at the desired angle of incidence. Such a diplexer, however, operates satisfactorily only over a narrow range of incidence angles, due to the walk-off effects associated with metallic mesh diplexers.
An alternative metallic mesh diplexer arrangement is disclosed in U.S. Pat. No. 2,636,125 issued to G. C. Southworth on Apr. 21, 1953. Waveguide structures are used to filter or purify a beam of electromagnetic waves for the purpose of restricting the beam to a desired frequency band. Moreover, within the transmission frequency band of the guide, the phase velocity for a wave of a given frequency is dependent upon the transverse dimension of the guide and increases as that transverse dimension decreases. Therefore, it is possible, by using a parallel assemblage of such guides, to build a structure through which the propagation velocity of a given frequency wave may be determined by the design of the structure.
An antenna system using the Southworth diplexer discussed hereinabove is disclosed in U.S. Pat. No. 2,870,444 issued to G. Broussaud on Jan. 20, 1959. This invention relates to an antenna capable of radiating or receiving simultaneously, two waves of different frequencies, with high efficiency and without any disturbing effect from one wave on the other. The Broussaud antenna comprises essentially a combination of two sources of radiation, positioned respectively on either side of a Southworth diplexer, serving respectively as a lens and a mirror for the two sources. In order for this structure to be capable of both transmitting and receiving, however, the antenna passbands must be separated by at least one octave.
In an alternative approach, multilayer stacks have been considered as a method of quasi-optical diplexing. One such design is disclosed in U.S. Pat. No. 3,698,001 issued to M. Koyama et al on Oct. 10, 1972. The koyama et al diplexer is designed to separate in reception the composed beams of high and low frequency groups, and conversely, in transmission to compose the separate beams of such high and low frequency groups. The diplexer comprises a plurality of laminated dielectric elements each having a thickness equal to one-fourth the wavelength of the central frequency of the high frequency group, and possessing as a whole at least two dielectric constants. However, the Koyama et al diplexer is not capable of separately detecting signal components having a broad frequency range and relatively close center frequencies.
The problem remaining in the prior art then, is to achieve quasi-optical diplexing over a wide angle of scan, without introducing the walk-off effects associated with metallic mesh diplexers.